Electroluminescent devices (hereinafter also referred to as EL devices) contain spaced electrodes separated by an electroluminescent medium that emits light in response to the application of an electrical potential difference across the electrodes. Through intensive investigations and a series of recent inventions organic electroluminescent devices of improved characteristics, both in terms of fabrication feasibility and operating performance, have been developed.
In current preferred forms, organic EL devices are comprised of an anode, an organic hole injecting and transporting zone in contact with the anode, an electron injecting and transporting zone forming a junction with the organic hole injecting and transporting zone, and a cathode in contact with the electron injecting and transporting zone. When an electrical potential is placed across the electrodes, holes and electrons are injected into the organic zones from the anode and cathode, respectively. Light emission results from hole-electron recombination within the device.
A class of organic EL devices that have exhibited highly desirable levels of efficiency and stability are those that have employed a metal oxinoid charge accepting compound to form the electron injecting and transporting zone of the organic EL device. The following patents and publications disclose the preparation and utilization of metal oxinoid compounds:
R-1. VanSlyke et al., U.S. Pat. No. 4,539,507, discloses in column 9, lines 14 to 16 inclusive, metal complexes of 8-hydroxyquinoline, where the metal is Zn, Al, Mg, or Li. In Example 9 the metal complex is bis(8-hydroxyquinolino)aluminum and in Example 10, bis(8-hydroxyquinolino)magnesium.
R-2. Tang et al., U.S. Pat. No. 4,769,292, discloses constructing an organic EL device in which the luminescent layer is comprised of a charge accepting host material and a fluorescent material. The host material can be chosen from among diarylbutadienes, stilbenes, optical brighteners, and metal oxinoid compounds, with the following being listed among preferred embodiments: aluminum trisoxine, magnesium bisoxine, bis[-benzo{f}-8-quinolino]zinc, indium trisoxine, aluminum tris(5-methyloxine), lithium oxine, gallium trisoxine, calcium bis(5-chlorooxine), poly[zinc(II)-bis(8-hydroxy-5-quinolinyl)methane, dilithium epindolidione, 1,4-diphenylbutadiene, 1,1,4,4-tetraphenylbutadiene, 4,4'-bis[5,7-di(t-pentyl-2-benzoxazolyl]stilbene, 2,5-bis[5,7-di(t-pentyl-2-benzoxazolyl]thiophene, 2,2'-(1,4-phenylene-divinylene)bisbenzothiazole, 4,4'-(2,2'-bis-thiazolylbiphenyl, 2,5-bis[5-.alpha.,.alpha.-dimethylbenzyl)-2-benzoxazolyl]thiophene, 2,5-bis[5,7-di(t-pentyl)-2-benzoxazolyl]-3,4-diphenylthiophene, and trans-stilbene.
R-3. VanSlyke et al., U.S. Pat. No. 4,720,432, discloses organic EL devices in which the organic hole injecting and transporting zone is comprised of a layer containing a hole injecting prophyrinic compound in contact with the anode and a layer containing a hole transporting aromatic tertiary amine interposed between the hole injecting layer and the electron injecting and transporting zone. The metal oxinoid charge accepting compounds are those disclosed to form the electron injecting and transporting zone in R-2.
R-4. Tang et al., U.S. Pat. No. 4,885,211, discloses organic EL devices with improved cathodes containing a metal oxinoid compound in the electron injecting and transporting zone. The metal oxinoid charge accepting compounds are those disclosed to form the electron injecting and transporting zone in R-2.
R-5. Perry et al., U.S. Pat. No. 4,950,950, discloses organic EL devices in which the hole injecting and transporting zone is comprised of (a) a layer containing a hole injecting porphyrinic compound in contact with the anode and (b) a layer containing a hole transporting silazane interposed between the hole injecting layer and the electron injecting and transporting zone. The metal oxinoid charge accepting compounds are those disclosed to form the electron injecting and transporting zone in R-2. Aluminum oxinate is set out in the Examples.
R-6. Kushi et al., "The Crystal and Molecular Structure of Bis(2-methyl-8-quinolinoato)aluminum(III)-.mu.-oxo-bis(2-methyl-8-quinolin olato)aluminum(III)", J. Amer. Chem. Soc., 92(1), pp. 91-96 (1970), discloses the preparation of the title and similar compounds.
Other related patents include the following:
R-7. VanSlyke et al., U.S. Pat. No. 5,059,862, discloses an internal junction organic electroluminescent device comprised of, in sequence, an anode, an organic hole injecting and transporting zone, an organic electron injecting and transporting zone forming a junction with the organic hole injecting and transporting zone, and a cathode comprised of a layer that contacts the organic electron injecting and transporting zone and contains a combination of magnesium and aluminum, with aluminum accounting for at least 80 percent of the cathode layer. The metal oxinoid charge accepting compounds are those disclosed to form the electron injecting and transporting zone in R-2.
R-8. Littman et al., U.S. Pat. No. 5,059,861, discloses an organic electroluminescent device comprised of, in sequence, a support, an anode, an organic electroluminescent medium, and cathode containing a plurality of metals other than alkali metals. The cathode is comprised of a capping layer containing at least one alkaline earth or rare earth metal and an electron injecting layer containing at least one metal having a work function of less than 4.0, but a higher work function than the alkaline earth or rare earth in the capping layer. The metal oxinoid charge accepting compounds are those disclosed to form the electron injecting and transporting zone in R-2.
R-9. VanSlyke, U.S. Pat. No. 5,047,687, discloses an organic electroluminescent device comprised of, in sequence, a support, an anode, an organic electroluminescent medium, and a cathode. The cathode is comprised of a layer containing a plurality of metals (other than alkali metals) having a work function of less than 4 eV. A protective layer overlies the cathode and is comprised of a mixture of at least one organic component of the organic electroluminescent medium and at least one metal having a work function in the range of from 4.0 to 4.5 and capable of being oxidized in the presence of ambient moisture. The metal oxinoid charge accepting compounds are those disclosed to form the electron injecting and transporting zone in R-2.
R-10. Scozzafava et al., U.S. Pat. No. 5,073,446, discloses an organic electroluminescent device comprised of, in sequence, an anode, an organic electroluminescent medium, and a cathode consisting essentially of a plurality of metals other than alkali metals, at least one of the metals having a work function less than that of indium. The cathode is comprised of a layer of fused metal particles containing at least 80 percent indium and having a mean diameter of less than 1 .mu.m and a coefficient of variation of less than 20 percent. The metal oxinoid charge compounds are those disclosed to form the electron injecting and transporting zone in R-2.
R-11. VanSlyke et al., U.S. Pat. No. 5,061,569, discloses an internal junction organic electroluminescent device comprised of, in sequence, an anode, an organic hole injecting and transporting zone comprised of a layer containing a hole transporting aromatic tertiary amine, an organic electron injecting and transporting zone, and a cathode. The hole transporting aromatic tertiary amine is comprised of at least two tertiary amine moieties and includes attached to a tertiary amine nitrogen atom an aromatic moiety containing at least two fused aromatic rings. The metal oxinoid charge accepting compounds are those disclosed to form the electron injecting and transporting zone in R-2.
R-12. VanSlyke et al., U.S. Pat. No. 5,150,006, discloses an internal junction organic EL device comprised of, in sequence, an anode, an organic hole injecting and transporting zone, an organic electron injecting and transporting zone, and a cathode. The organic electron injecting and transporting zone is comprised of an electron injecting layer in contact with the cathode and, interposed between the electron injecting layer and the organic hole injecting and transporting zone, a blue emitting luminescent layer comprised of an aluminum chelate containing a phenolato ligand and two R.sup.s -8-quinolinolato ligands, where R.sup.s substituents are chosen to block the attachment of more than two substituted 8-quinolinolato ring nuclei to the aluminum atom. The presence of the phenolato ligand shifts device emission to the blue region of the spectrum and increases emission efficiency. Device emission at even shorter blue wavelengths and increased operating stability can be realized by the incorporation of a pentacarbocyclic aromatic fluorescent dye.
R-13. VanSlyke, U.S. Pat. No. 5,151,629, discloses an internal junction organic EL device comprised of, in sequence, an anode, an organic hole injecting and transporting zone, an organic electron injecting and transporting zone, and a cathode. The organic electron injecting and transporting zone is comprised of an electron injecting layer in contact with the cathode and, interposed between the electron interjecting layer and the organic hole injecting and transporting zone, a blue emitting luminescent layer comprised of bis (R.sup.s -8-quinolinolato) aluminum (III)-.mu.-oxo-bis (R.sup.s -8-quinolinolato) Aluminum (III), where R.sup.s substituents are chosen to block the attachment of more than two substituted 8-quinolinolato ring nuclei to any one aluminum atom. Increased operating stability and device emission at even shorter blue wavelengths can be realized by the incorporation of a pentacarbocyclic aromatic fluorescent dye.
R-14. Bryan et al., U.S. Pat. No. 5,141,671, discloses novel light emitting compositions containing a mixed ligand aluminum chelate having a phenolato ligand and two 8-quinolinolato ligands that are ring substituted to prevent the attachment of three of these ligands to a single aluminum atom. The mixed ligand aluminum chelate can be present in combination with a fluorescent dye in the luminescent composition.
Electroluminescent (EL) devices capable of full color display are highly desirable. Their process of manufacture, however, is very difficult, requiring precise patterning to produce pixels of different colors. The fabrication of inorganic EL devices presents particular problems, requiring high processing temperatures. A white light emitting inorganic EL device containing an SrS:Ce,K,Eu phosphor layer has been described by Tanaka et al. in SID 89 Digest, 1989, pages 321-324. Reportedly, this device would allow full color display when used with stacked red, green, and blue colored interference filters. The preparation of the SrS prosphor film, however, required process temperatures of 400.degree.-500.degree. C.
U.S. Pat. No. 4,983,469 discloses an inorganic EL element containing a luminescent film for emitting white light that includes three contiguous layers, one layer with no luminescent center being interposed between two luminescent layers having different luminescent centers.